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"Since the valve must move a distance equal to the outside lap before admission can take place, it is evident that the eccentric can no longer be at right angles to the crank at the beginning of the stroke, but must be ahead of the right—angle point by an amount equal to AOC." —Derr, 1911

Valve with Lap of Steam Engine at Extreme Left with Exhaust Ports Open

"Since the valve must move a distance equal to the outside lap before admission can take place, it is…

"The relative positions of crank, eccentric and valve when the exhaust closes on the crank end and compression begins. Between these two positions the steam is expanding in the head end and exhausting from the crank end." —Derr, 1911

Steam Engine Valve with Lap Steam Where Slide Valve is Closed

"The relative positions of crank, eccentric and valve when the exhaust closes on the crank end and compression…

"While the crank is moving from the position, steam is being admitted to the head end and being exhausted from the crank end. The inside lap being less than the outside lap, causes the exhaust to continue longer than the admission." —Derr, 1911

Steam Engine Valve with Lap Where Steam is Admitted to the Slide Valve

"While the crank is moving from the position, steam is being admitted to the head end and being exhausted…

An illustration of the valve with lap at the end of the crank end steam compression. The piston will restart the compression cycle of at the head of the piston. As the piston moves, the sliding valve in the top compartment will fill with steam and closes.

Steam Engine Valve with Lap Steam Where Slide Valve is in Open Position

An illustration of the valve with lap at the end of the crank end steam compression. The piston will…

"If the inside lap is increased, the valve must move farther before released occurs and the crank angle would be greater than illustrated. On the return of the valve to the dotted position, the port will close earlier and make an earlier compression. Decreasing inside lap will cause earlier release and later compression." —Derr, 1911

Steam Engine Valve with Lap Steam Where Slide Valve Moved to Open Position

"If the inside lap is increased, the valve must move farther before released occurs and the crank angle…

"The maximum displacement of the valve is attained when the eccentric is horizontal. In this position both the steam and exhaust ports are wide open, and any further motion motion of the piston will cause the valve to move toward its mid position." —Derr, 1911

Valve with Lap of Steam Engine Where Steam Enters the Slide Valve Compartment

"The maximum displacement of the valve is attained when the eccentric is horizontal. In this position…

"The amount that the valve overlaps the steam ports is called the lap of the valve. It will at once be seen that both the admission and exhaust ports may remain closed during a part of the stroke, thus making expansion and compression possible." —Derr, 1911

Slide Valve of Steam Engine

"The amount that the valve overlaps the steam ports is called the lap of the valve. It will at once…